1. Field of the Invention
The present invention relates to a system and method for monitoring the mechanical condition of a reciprocating compressor.
2. Background Art
The production of low density polyethylene requires the use of very high pressures. In fact, polymerization pressures can reach as high as 50,000 pounds per square inch (psi). To achieve these pressures, high pressure reciprocating compressors, or hypercompressors, are used. Hypercompressors typically use “packed-plunger” cylinders of either “pressure-wrapped” or “tie-rod” construction. Monitoring the mechanical condition of the cylinder components during operation of the compressor is important for determining maintenance requirements.
An important parameter in monitoring the mechanical condition of a reciprocating compressor is the internal pressure. By monitoring the internal pressure of the cylinder, several parameters can be analyzed to determine if any of the cylinder components are deficient. By identifying a deficiency, preventative maintenance can be scheduled, and performed at a convenient time to minimize production downtime. The internal pressure of a cylinder in a hypercompressor may be difficult to obtain, since the ultrahigh pressure within the cylinder prohibits a direct measurement. Thus, a need exists for a non-intrusive pressure measurement technique that will provide information about the internal pressure of a cylinder in a hypercompressor that will facilitate monitoring the mechanical condition of the compressor.
One type of non-intrusive pressure measurement is described in U.S. Pat. No. 6,494,343, issued to McManus et al. on Dec. 17, 2002. McManus et al. discusses the use of a strain responsive sensor disposed on a exterior portion of a pressure vessel. Known relationships between the stress and strain of a thin-walled pressure vessel are then used to calculate the internal pressure of the vessel based on the external strain measured by the strain gauge. One limitation of the system described in McManus et al. is that the thin-walled pressure vessel equations are not applicable to a relatively thick-walled cylinder, such as a packed-plunger cylinder used in a hypercompressor. In addition, the pressure within the hypercompressor cylinder is not constant, but rather, it varies cyclically based on the reciprocating motions of a plunger. Therefore, a need still exists for a non-intrusive pressure monitoring system and method that can be effectively used with a packed-plunger cylinder in a hypercompressor.
U.S. Pat. No. 4,456,963, issued to Wiggins on Jun. 26, 1984, describes an apparatus and method for measuring performance characteristics of a reciprocating piston engine or compressor. The Wiggins apparatus uses a pressure transducer that is attached to the engine/compressor cylinder through an indicator valve. The pressure transducer may be a strain gauge type transducer that provides a voltage signal to an output device, such as an oscilloscope. Rather than calibrating output from the transducer with a known internal pressure, the Wiggins apparatus uses a known relationship between the full scale pressure range of the pressure transducer and the sensitivity of the pressure transducer. Once converted, the output from the pressure transducer may be displayed with respect to a crankshaft angle of the engine/compressor.
One limitation of the Wiggins apparatus and method is that it does not provide for a non-intrusive pressure measurement, which is desirable when working with hypercompressors. The use of an indicator valve in a compressor cylinder, such as described in Wiggins, would not only create a potential leak path, but could add significantly to the cylinder stress. Therefore, a need still exists for a system and method for monitoring the mechanical condition of a hypercompressor, and in particular for non-intrusively monitoring the pressure of a packed-plunger cylinder in the compressor.